A DPF is described in JP-A-56-129020. After a plurality of porous honeycomb segments made of ceramic such as silicon carbide are bonded together by means of a bonding layer of segments, the periphery is covered with a coating material layer. The DPF is disposed in an exhaust gas system of a diesel engine and used for purifying exhaust gas.
Each of the honeycomb segments in the aforementioned DPF is partitioned by porous partition walls and has a large number of through channels extending through in the axial direction. Adjacent through channels have alternately plugged ends. That is, the through channel is open in one side end portion and plugged in the other side end portion. Another through channel adjacent to the through channel is plugged in the other side end portion and open in the one side end portion.
In the DPF having such a structure, exhaust gas flows in from the open end portion of through channels, passes through the porous partition walls, and discharged from the other through channels. When the exhaust gas passes through the partition walls, particulate matter typified by soot in exhaust gas is trapped by the partition walls to purify the exhaust gas.
In such a DPF, by continuous purification of exhaust gas, soot deposits in the through channels, and pressure loss increases with the lapse of time to decrease purification efficiency. Therefore, it is necessary to perform regeneration, where soot is removed by combustion. It has been known that, by the soot combustion heat upon regeneration, temperature becomes highest in the central portion on the exhaust gas outlet side in each of the honeycomb segments (see the publication “SAE Technical Series 870010” published in February, 1983). During such regeneration of honeycomb segments, the maximum temperature on the outlet side becomes durable temperature or more of the honeycomb segments to cause problems of crack generation in a honeycomb segment and/or deterioration of a loaded catalyst.
Above all, a DPF having a segment structure has a problem in controlling the maximum temperature upon regeneration. That is, since, in a DPF having a segment structure, soot is not deposited in the bonding portion, and the bonding portion has low thermal conductivity and high thermal capacity, as shown in FIGS. 9A to 9C, the outer peripheral portion of the segment 102 has low temperature to form a heat spot in the center of the segment 102. Therefore, the peculiar temperature distribution may cause deformation due to thermal expansion in the outlet end face of a segment and/or application of tensile stress to cause a problem of easy crack generation in the outlet end face.
By the way, a trial of solving the aforementioned problems has conventionally been made, such as adjustment of temperature so that the inlet temperature of a DPF upon regeneration is in a certain level or less, controlling the oxygen temperature in air supplied and/or the flow rate of the air supplied upon regeneration, and adjustment of regeneration timing lest the amount of soot deposited in a honeycomb segment should be in a certain level or more. For example, there are the following Patent Documents 2 to 4.
In the Patent Document 2, to solve the problems is tried by arranging a partial plug in the vicinity of opening portion to suppress excessive pressure loss generation and to inhibit a crack or melt due to deposition of particulate matter. However, by the partial plug 99 shown in the Patent Document 2, for example, as shown in FIG. 10, PM is generally deposited to cause clogging in the PM before regeneration, and, inversely, PM density in the vicinity of the partial plug rises to make temperature higher than the ordinary plugging portion upon regeneration. Therefore, the maximum temperature upon regeneration cannot be suppressed sufficiently, and it does not serve as a sufficient countermeasure against the problems of a deformation due to thermal expansion of the segment and easily crack generation. Above all, as to the problem caused by the peculiar temperature and stress distributions because of the aforementioned segment structure, the Patent Document 2 discloses no technical concept regarding how to control the peculiar temperature and stress distributions which may be caused in each segment because of unitary formation is shown, and the problems have not been solved.
The Patent Document 3 discloses a structure where at least part of the plugging portions have a through-hole formed therein, which seems to have an effect to some extent. However, the trapping efficiency is prone to vary due to deposition of particulate matter, and the Patent Document 3 shows a unitarily formed honeycomb structure. Therefore, it is not sufficient for the problems of the DPFs having a segment structure.
In the Patent Document 4, a plurality of honeycomb-shaped carrier substrates each having a plurality of cells extending in almost parallel with the axial direction are arranged in series in the exhaust gas flow passage to constitute an exhaust gas purification apparatus. In this constitution where two or more substrates are arranged in series, as the regeneration is repeated, the PM deposited in a downtown DPF cannot be combusted completely to make pressure loss very high, and extraordinary combustion may be caused in the downstream DPF upon regeneration after repetition. Thus, the problems have not been solved.
As described above, none of the Patent Documents 1 to 4 shows a sufficient countermeasure against the problems caused upon regeneration of a honeycomb segment, above all, the problems upon regeneration in a segment structure, and further improvement is required.    Patent Document 1: JP-A-56-129020    Patent Document 2: JP-A-2002-256842    Patent Document 3: JP-A-2004-130229    Patent Document 4: JP-B-3874258    Non-Patent Document 1: Publication “SAE Technical Series 870010”